lguest: Fix Malicious Guest GDT Host Crash

If a Guest makes hypercall which sets a GDT entry to not present, we
currently set any segment registers using that GDT entry to 0.
Unfortunately, this is not sufficient: there are other ways of
altering GDT entries which will cause a fault.

The correct solution to do what Linux does: let them set any GDT value
they want and handle the #GP when popping causes a fault.  This has
the added benefit of making our Switcher slightly more robust in the
case of any other bugs which cause it to fault.

We kill the Guest if it causes a fault in the Switcher: it's the
Guest's responsibility to make sure it's not using segments when it
changes them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 6 deletions

diff --git a/drivers/lguest/switcher.S b/drivers/lguest/switcher.S
index d418179ea6b5..7c9c230cc845 100644
--- a/drivers/lguest/switcher.S
+++ b/drivers/lguest/switcher.S
@@ -47,6 +47,7 @@
 // Down here in the depths of assembler code.
 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
+#include <asm/page.h>
 #include "lg.h"
 
 // We mark the start of the code to copy
@@ -182,13 +183,15 @@ ENTRY(switch_to_guest)
         movl    $(LGUEST_DS), %eax;                                     \
         movl    %eax, %ds;                                              \
         /* So where are we?  Which CPU, which struct?                   \
-         * The stack is our clue: our TSS sets                          \
-         * It at the end of "struct lguest_pages"                       \
-         * And we then pushed and pushed and pushed Guest regs:         \
-         * Now stack points atop the "struct lguest_regs".              \
-         * Subtract that offset, and we find our struct. */             \
+         * The stack is our clue: our TSS starts                        \
+         * It at the end of "struct lguest_pages".                      \
+         * Or we may have stumbled while restoring                      \
+         * Our Guest segment regs while in switch_to_guest,             \
+         * The fault pushed atop that part-unwound stack.               \
+         * If we round the stack down to the page start                 \
+         * We're at the start of "struct lguest_pages". */              \
         movl    %esp, %eax;                                             \
-        subl    $LGUEST_PAGES_regs, %eax;                               \
+        andl    $(~(1 << PAGE_SHIFT - 1)), %eax;                        \
         /* Save our trap number: the switch will obscure it             \
          * (The Guest regs are not mapped here in the Host)             \
          * %ebx holds it safe for deliver_to_host */                    \